Strip holder for use in a test strip meter

ABSTRACT

Test strip holders for use with test strip meters are provided. The subject test strip holders include at least an opening and a lip associated with the opening. The lip element of the subject holders is capable of forming a liquid seal with the upper surface of a test strip upon insertion of the test strip into the opening. In many embodiments, the strip holder is configured to at least partially encompass a sample application region of a test strip upon insertion of the strip into the opening. Also provided are meters on which the subject test holders are present, as well as methods for using the same.

FIELD OF THE INVENTION

[0001] The field of this invention is fluidic medical diagnostic devicesfor measuring the concentration of an analyte in or a property of abiological fluid.

BACKGROUND OF THE INVENTION

[0002] A variety of medical diagnostic procedures involve tests onbiological fluids, such as blood, urine, or saliva, and are based on achange in a physical characteristic of such a fluid or an element of thefluid, such as blood serum. The characteristic can be an electrical,magnetic, fluidic, or optical property. When an optical property ismonitored, these procedures may make use of a transparent or translucentdevice to contain the biological fluid and a reagent. A change in lightabsorption of the fluid can be related to an analyte concentration in,or property of, the fluid.

[0003] A growing number of assay formats employ a disposable test strip,fluidic device or card which is used in conjunction with a meter. Thedisposable fluid device receives the sample to be assayed and includesany reagents necessary for the assay to be conducted. The test stripalso typically includes one or more flow paths through which the sampleflows during the assay.

[0004] As mentioned above, these test strips are typically used inconjunction with a meter which is capable of receiving a signaloriginated in a measurement area of the card. To receive the signal fromthe measurement area, the test strip is generally inserted into anopening in the meter so that at least the measurement area of the teststrip is present inside the meter. Examples of assay systems that aremade up of these types of disposable test strips and meters may be foundin application Ser. No. 09/333765, filed Jun. 15, 1999; and Ser. No.09/356248, filed Jul. 16, 1999; the disclosures of which are hereinincorporated by reference.

[0005] Because the test strip is inserted into the meter in such assaysystems, there is necessarily an opening in the meter for receiving thetest strip. This opening is potentially a means for interferingmaterials to enter inside the meter and adversely interact with theinternal workings of the meter.

[0006] As such, there is a need for the development of a device that iscapable of providing entry of a test card or strip into a meter buteffectively keeps the inside of the meter free of interfering orcontaminating agents.

[0007] Relevant Literature

[0008] References of interest include: U.S. Pat. Nos. 3,620,676;3,640,267; 4,088,448; 4,426,451; 4,868,129; 5,104,813; 5,230,866;5,700,695; 5,736,404; 5,208,163; and European Patent Application EP 0803 288.

SUMMARY OF THE INVENTION

[0009] Test strip holders for use with test strip meters are provided.The subject test strip holders include at least an opening and a lipassociated with the opening. The lip element of the subject holders iscapable of forming a liquid seal with the upper surface of a test stripupon insertion of the test strip into the opening. In many embodiments,the strip holder is configured to at least partially encompass a sampleapplication region of a test strip upon insertion of the strip into theopening. Also provided are meters on which the subject test holders arepresent, as well as methods for using the same.

BRIEF DESCRIPTION OF THE FIGURES

[0010]FIG. 1 is a plan view of a bladder including test strip of asystem with which the subject strip holders may be employed.

[0011]FIG. 2 is an exploded view of the device of FIG. 1.

[0012]FIG. 3 is a perspective view of the device of FIG. 1.

[0013]FIG. 4 is a schematic of a meter that includes a strip holderaccording to the subject invention.

[0014]FIG. 4A depicts an alternative embodiment of an element of themeter of FIG. 4.

[0015]FIG. 5 is a graph of data that is used to determine PT time.

[0016]FIG. 6A provides an overhead view of a meter device with aremovable strip holder according to the subject invention placed overthe opening of the meter. FIG. 6B shows a cross section view of the teststrip holder shown in FIG. 6A, where the cross-sectional view is takenalong Section A-A as shown in FIG. 6A. FIG. 6C provides an expanded viewof FIG. 6B.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

[0017] Test strip holders for use with test strip meters are provided.The subject test strip holders include at least an opening and a lipassociated with the opening. The lip element of the subject holders iscapable of forming a liquid seal with the upper surface of a test stripupon insertion of the test strip into the opening. In many embodiments,the strip holder is configured to at least partially encompass a sampleapplication region of a test strip upon insertion of the strip into theopening. Also provided are meters on which the subject test holders arepresent, as well as methods for using the same. In further describingthe subject invention, the subject test strip holders will be discussedfirst in greater detail, both generally and in terms of the figures,followed by a review of a representative meter/test strip system inwhich the subject test strip holders find use, as well as methods forusing the same.

[0018] Before the subject invention is described further, it is to beunderstood that the invention is not limited to the particularembodiments of the invention described below, as variations of theparticular embodiments may be made and still fall within the scope ofthe appended claims. It is also to be understood that the terminologyemployed is for the purpose of describing particular embodiments, and isnot intended to be limiting. Instead, the scope of the present inventionwill be established by the appended claims.

[0019] In this specification and the appended claims, singularreferences include the plural, unless the context clearly dictatesotherwise. Unless defined otherwise, all technical and scientific termsused herein have the same meaning as commonly understood to one ofordinary skill in the art to which this invention belongs.

[0020] Test Strip Holders

[0021] As summarized above, the subject test strip holders areconfigured to be used with meters, where the meters have an opening forreceiving at least a portion of test card or strip, e.g. a fluidic testcard or device. Representative meters and test strips with which thesubject holders find use are disclosed in U.S. patent application Ser.No. 09/333765, filed Jun. 15, 1999; and Ser. No. 09/356248, filed Jul.16, 1999; the disclosures of which are herein incorporated by reference.In many embodiments, the strip holders of the subject invention arereadily removable from the meters with which they are used, i.e. theyare not bolted, screwed or otherwise “permanently” affixed to the meter,e.g. they can be snapped onto and off of the meter, etc.

[0022] The subject strip holders include an opening for receiving a teststrip. The opening is dimensioned so that the test strip is readilymovable through the opening, but extra space on either side of the stripis kept to a minimum. While the particular dimensions of the opening mayvary depending on the particular meter and fluidic test strip with whichthe holder is to be used, in many embodiments the opening has a widthranging from about 0.25″ to 2″, usually from about 0.8″ to 1.3″ and moreusually from about 1.055″ to 1.075″; and a height ranging from about0.015″ to 0.125″, usually from about 0.02″ to 0.06″ and more usuallyfrom about 0.025″ to 0.035″.

[0023] The subject strip holder is further characterized by having a lipor analogous element that is capable of contacting the surface of a teststrip when inserted into the opening and forming a liquid seal. Byliquid seal is meant that any space existing between the upper surfaceof a test strip inserted into the opening and the lip ranges from about0.000″ to 0.002″, usually from about 0.0005″ to 0.0015″, and moreusually from about 0.0009″ to 0.0011″, so that liquid is substantiallyprevented from entering the internal portion of the meter through theopening of the test strip holder that is placed over the opening of themeter.

[0024] To provide for this liquid seal, the subject strip holdersfurther include a raised element or bump that contacts the bottomsurface of a test strip when the test strip is inserted into the device.This raised bump or contact is generally configured to contact the teststrip beneath the sample application region of the test strip when thetest strip is inserted through the strip holder.

[0025] In those embodiments where the test strip holder includes theabove discussed raised bump or element, the force applied by the abovedescribed lip element and the raised bump is substantially the same oridentical in location, magnitude and opposite in direction. Depending onthe particular embodiment, the force applied by the lip and/or theraised element or bump may range from about 0.01 lb to 0.2 lb, usuallyfrom about 0.01 lb to 0.1 lb and more usually from about 0.01 lb to 0.05lb.

[0026] In many embodiments, the strip holder is further characterized bybeing configured so that a sample application region of a test stripinserted through the opening of the holder is at least partiallyencompassed, surrounded or encircled by the lip element of the holder.As such, the lip element of the holder may be configured as a partialcircle (as shown in the figures), a partial square, triangle etc., whichserves to at least partially encompass the sample application region ofthe test strip when inserted into the opening of the holder.

[0027] Turning now to the figures, FIG. 6A provides an overhead view ofa meter device 60 with a removable strip holder 62 according to thesubject invention placed over the opening of the meter. Also shown inFIG. 6A is test strip 10 showing sample application port 12. As is shownin FIG. 6A, the strip holder 62 is configured to at least partiallyencompass the sample application port 12 by forming a semi-circle aroundthe sample application port 12. FIG. 6B shows a cross sectional view ofthe test strip holder shown in FIG. 6A, where the cross-sectional viewis taken along Section A-A as shown in FIG. 6A. FIG. 6C provides a blowup view of the view shown in FIG. 6B. In FIG. 6C, test strip 12 isinserted into test strip holder 62 and meter 60 in the direction ofarrow Y. Lip element 66 of test strip holder 62 presses down on teststrip 12 to form a liquid seal at the contact point of the lip elementand the upper surface of the strip, while raised element or bump 64pushes upward on the bottom of the strip with a substantially equal, ifnot identical force.

[0028] The subject test strip holders may be fabricated from anyconvenient material, where suitable materials include: plastic andmetals. The test strip holders may be fabricated using any convenientprotocol, where representative protocols include machining, injectionmolding, compression molding, casting and the like.

[0029] Systems

[0030] The above described strip holders find use with systems thatinclude fluidic devices or test strips and meters, as described below.

[0031] Test Strips

[0032] The fluidic test strips of the systems with which the subjectstrip holders find use are fluidic devices that generally include asample application area; a bladder, to create a suction force to drawthe sample into the device; a measurement area, in which the sample mayundergo a change in an optical parameter, such as light scattering; anda stop junction to precisely stop flow after filling the measurementarea. Preferably, the devices are substantially transparent over themeasurement area, so that the area can be illuminated by a light sourceon one side and the transmitted light measured on the opposite side.

[0033] A representative bladder including test strip with which thesubject strip holders find use is shown in FIGS. 1, 2 and 3. FIG. 1provides a plan view of a test strip 10, while FIG. 2 provides anexploded view and FIG. 3 provides a perspective view of the samerepresentative test strip. Sample is applied to sample port 12 afterbladder 14 has been compressed. Clearly, the region of layer 26 and/orlayer 28 that adjoins the cutout for bladder 14 must be resilient, topermit bladder 14 to be compressed. Polyester of about 0.1 mm thicknesshas suitable resilience and springiness. Preferably, top layer 26 has athickness of about 0.125 mm, bottom layer 28 about 0.100 mm. When thebladder is released, suction draws sample through channel 16 tomeasurement area 18, which preferably contains a reagent 20. In order toensure that measurement area 18 can be filled with sample, the volume ofbladder 14 is preferably at least about equal to the combined volume ofchannel 16 and measurement area 18. If measurement area 18 is to beilluminated from below, layer 28 must be transparent where it adjoinsmeasurement area 18.

[0034] As shown in FIGS. 1, 2, and 3, stop junction 22 adjoins bladder14 and measurement area 18; however, a continuation of channel 16 may beon either or both sides of stop junction 22, separating the stopjunction from measurement area 18 and/or bladder 14. When the samplereaches stop junction 22, sample flow stops. The principle of operationof stop junctions is described in U.S. Pat. No. 5,230,866; incorporatedherein by reference.

[0035] As shown in FIG. 2, all the above elements are formed by cutoutsin intermediate layer 24, sandwiched between top layer 26 and bottomlayer 28. Preferably, layer 24 is double-sided adhesive tape. Stopjunction 22 is formed by an additional cutout in layer 26 and/or 28,aligned with the cutout in layer 24 and sealed with sealing layer 30and/or 32. Preferably, as shown, the stop junction comprises cutouts inboth layers 26 and 28, with sealing layers 30 and 32. Each cutout forstop junction 22 is at least as wide as channel 16. Also shown in FIG. 2is an optional filter 12A to cover sample port 12. The filter mayseparate out red blood cells from a whole blood sample and/or maycontain a reagent to interact with the blood to provide additionalinformation. A suitable filter comprises an anisotropic membrane,preferably a polysulfone membrane of the type available from SpectralDiagnostics, Inc., Toronto, Canada. Optional reflector 18A may be on, oradjacent to, a surface of layer 26 and positioned over measurement area18. If the reflector is present, the device becomes a transflectancedevice.

[0036] The test strip pictured in FIG. 2 and described above ispreferably formed by laminating thermoplastic sheets 26 and 28 to athermoplastic intermediate layer 24 that has adhesive on both of itssurfaces. The cutouts that form the elements shown in FIG. 1 may beformed, for example, by laser- or die-cutting of layers 24, 26, and 28.Alternatively, the device can be formed of molded plastic. Preferably,the surface of sheet 28 is hydrophilic. (Film 9962, available from 3M,St. Paul, Minn.) However, the surfaces do not need to be hydrophilic,because the sample fluid will fill the device without capillary forces.Thus, sheets 26 and 28 may be untreated polyester or other thermoplasticsheet, well known in the art. Similarly, since gravity is not involvedin filling, the device can be used in any orientation. Unlike capillaryfill devices that have vent holes through which sample could leak, thesetypes of devices vent through the sample port before sample is applied,which means that the part of the strip that is first inserted into themeter is without an opening, reducing the risk of contamination.

[0037] Other test strip configurations are also possible, where suchalternative device configurations include those that have: a bypasschannel; multiple parallel measurement areas; and or multiple in seriesmeasurement areas, etc. In addition, the above described laminatedstructures can be adapted to injection molded structures. A variety ofalternative fluidic devices are described in co-pending application Ser.No. 09/333765, filed Jun. 15, 1999; and Ser. No. 09/356248, filed Jul.16, 1999; the disclosures of which are herein incorporated by reference.

[0038] Meters

[0039] The subject test strip holders find use with meters, generallyautomated meters, that are designed for use with the above describedtest strip holders. A representative meter is depicted in FIG. 4,wherein a representative test strip 10 is inserted into the meter. Themeter shown in FIG. 4 includes strip detector 40 (made up of LED 40 aand detector 40 b), sample detector 42 (made up of light source 42 a anddetector 42 b), measurement system 44 (made up of LED 44 a and detector44 b), and optional heater 46. The device further includes a bladderactuator 48. The bladder actuator is, in many embodiments, actuated bythe strip detector 40 and the sample detector 42, such that when a stripis inserted into the meter and detected by the strip detector, thebladder actuator is depressed, and when the sample is added to thefluidic device or strip inserted into the meter, the bladder actuator iswithdrawn so as to decompress the bladder and concomitantly pull sampleinto the measurement area of the device via the resultant negativepressure conditions. Also present is a meter display 50 that providesfor an interface with the user. Finally, the meter is shown with teststrip holder 62 positioned over the opening of the meter and strip 10 isinserted into the opening of the test strip holder 62.

[0040] Methods of Use

[0041] The above described fluidic device/meter systems that include thesubject test strip holders are suitable for use in a variety ofanalytical tests of biological fluids, such as determining biochemicalor hematological characteristics, or measuring the concentration in suchfluids of analytes such as proteins, hormones, carbohydrates, lipids,drugs, toxins, gases, electrolytes, etc. The procedures for performingthese tests have been described in the literature. Among the tests, andwhere they are described, are the following: (1) Chromogenic Factor XIIaAssay (and other clotting factors as well): Rand, M. D. et al., Blood,88, 3432 (1996); (2) Factor X Assay: Bick, R. L. Disorders of Thrombosisand Hemostasis: Clinical and Laboratory Practice. Chicago, ASCP Press,1992.; (3) DRVVT (Dilute Russells Viper Venom Test): Exner, T. et al.,Blood Coag. Fibrinol., 1, 259 (1990); (4) Immunonephelometric andImmunoturbidimetric Assays for Proteins: Whicher, J. T., CRC Crit. Rev.Clin Lab Sci. 18:213 (1983); (5) TPA Assay: Mann, K. G., et al., Blood,76, 755, (1990).; and Hartshorn, J. N. et al., Blood, 78, 833 (1991);(6) APTT (Activated Partial Thromboplastin Time Assay): Proctor, R. R.and Rapaport, S. I. Amer. J. Clin. Path, 36, 212 (1961); Brandt, J. T.and Triplett, D. A. Amer. J. Clin. Path., 76, 530 (1981); and Kelsey, P.R. Thromb. Haemost. 52, 172 (1984); (7) HbAlc Assay (GlycosylatedHemoglobin Assay): Nicol, D. J. et al., Clin. Chem. 29, 1694 (1983); (8)Total Hemoglobin: Schneck et al., Clinical Chem., 32/33, 526 (1986); andU.S. Pat. No. 4,088,448; (9) Factor Xa: Vinazzer, H., Proc. Symp. Dtsch.Ges. Klin. Chem., 203 (1977), ed. By Witt, I; (10) Colorimetric Assayfor Nitric Oxide: Schmidt, H. H., et al., Biochemica, 2, 22 (1995).

[0042] The above described fluid device/meter systems are particularlywell suited for measuring blood-clotting time—“prothrombin time” or “PTtime,” as more fully described in application Ser. No. 09/333765, filedJun. 15, 1999; and Ser. No. 09/356248, filed Jul. 16, 1999; thedisclosures of which are herein incorporated by reference. Themodifications needed to adapt the device for applications such as thoselisted above require no more than routine experimentation.

[0043] In using the above systems that include the subject test stripholders, the first step the user performs is to turn on the meter,thereby energizing strip detector 40, sample detector 42, measurementsystem 44, and optional heater 46. The second step is to insert thestrip. The strip is inserted through the opening of the test stripholder 62 and into the device. A liquid seal is formed at the contactpoint between the strip holder and the upper surface of test strip 10.Preferably, the strip is not transparent over at least a part of itsarea, so that an inserted strip will block the illumination by LED 40 aof detector 40 b. (More preferably, the intermediate layer is formed ofa non-transparent material, so that background light does not entermeasurement system 44.) Detector 40 b thereby senses that a strip hasbeen inserted and triggers bladder actuator 48 to compress bladder 14. Ameter display 50 then directs the user to apply a sample to sample port12 as the third and last step the user must perform to initiate themeasurement sequence. The empty sample port is reflective. When a sampleis introduced into the sample port, it absorbs light from LED 42 a andthereby reduces the light that is reflected to detector 42 b. Thatreduction in light, in turn, signals bladder actuator 48 to releasebladder 14. The resultant suction in channel 16 draws sample throughmeasurement area 18 to stop junction 22. Light from LED 44 a passesthrough measurement area 18, and detector 44 b monitors the lighttransmitted through the sample as it is clotting. Analysis of thetransmitted light as a function of time (as described below) permits acalculation of the PT time, which is displayed on the meter display 50.Preferably, sample temperature is maintained at about 37° C. by heater46.

[0044] As described above, the detector senses a sample in sample port12, simply by detecting a reduction in (specular) reflection of a lightsignal that is emitted by 42 a and detected by 42 b. However, thatsimple system cannot easily distinguish between a whole blood sample andsome other liquid (e.g., blood serum) placed in the sample port in erroror, even, an object (e.g., a finger) that can approach sample port 12and cause the system to erroneously conclude that a proper sample hasbeen applied. To avoid this type of error, another embodiment measuresdiffuse reflection from the sample port. This embodiment appears in FIG.4A, which shows detector 42 b positioned normal to the plane of strip10. With the arrangement shown in FIG. 4A, if a whole blood sample hasbeen applied to sample port 12, the signal detected by 42 b increasesabruptly, because of scattering in the blood sample, then decreases,because of rouleaux formation. The detector system 42 is thus programmedto require that type of signal before causing gimbaled bladder actuator48 to release bladder 14. The delay of several seconds in releasingbladder 14 does not substantially affect the readings described below

[0045]FIG. 5 depicts a typical “clot signature” curve in which thecurrent from detector 44 b is plotted as a function of time. Blood isfirst detected in the measurement area by 44 b at time 1. In the timeinterval A, between points 1 and 2, the blood fills the measurementarea. The reduction in current during that time interval is due to lightscattered by red cells and is thus an approximate measure of thehematocrit. At point 2, sample has filled the measurement area and is atrest, its movement having been stopped by the stop junction. The redcells begin to stack up like coins (rouleaux formation). The rouleauxeffect allows increasing light transmission through the sample (and lessscattering) in the time interval between points 2 and 3. At point 3,clot formation ends rouleaux formation and transmission through thesample reaches a maximum. The PT time can be calculated from theinterval B between points 1 and 3 or between 2 and 3. Thereafter, bloodchanges state from liquid to a semi-solid gel, with a correspondingreduction in light transmission. The reduction in current C between themaximum 3 and endpoint 4 correlates with fibrinogen in the sample.

[0046] It is evident from the above results and discussion that thesubject test strip holders provide for a simple and convenient way tokeep contaminating fluids and other agents out the internal compartmentsof a meter device during use. As such, the subject invention representsa significant contribution to the art.

[0047] All publications and patents cited in this specification areherein incorporated by reference as if each individual publication orpatent were specifically and individually indicated to be incorporatedby reference. The citation of any publication is for its disclosureprior to the filing date and should not be construed as an admissionthat the present invention is not entitled to antedate such publicationby virtue of prior invention.

[0048] Although the foregoing invention has been described in somedetail by way of illustration and example for purposes of clarity ofunderstanding, it is readily apparent to those of ordinary skill in theart in light of the teachings of this invention that certain changes andmodifications may be made thereto without departing from the spirit orscope of the appended claims.

What is claimed is:
 1. A strip holder for use with a meter, said stripholder comprising: (a) an opening for receiving a test strip; and (b) alip associated with said opening, wherein said lip forms a liquid sealwith the upper surface of said test strip when inserted into saidopening.
 2. The strip holder according to claim 1, wherein said stripholder is readily removable from said meter.
 3. The strip holderaccording to claim 1, wherein said strip holder is configured to atleast partially encompass a sample application region of said test stripwhen inserted into said opening.
 4. The strip holder according to claim1, wherein said strip holder further comprises a raised bump thatcontacts the bottom surface of said test strip when inserted into saidopening.
 5. The strip holder according to claim 4, wherein the forceapplied by said lip on said top surface of said strip inserted into saidopening is substantially equal, opposite in direction and at the samelocation as the force applied by said raised bump on said bottom surfaceof said strip inserted into said opening.
 6. The strip holder accordingto claim 5, wherein said applied forces range in magnitude from about0.01 lb to 0.05 lb.
 7. A readily removable strip holder for use with atest strip meter, said strip holder comprising: (a) an opening forreceiving a test strip; (b) a lip associated with said opening, whereinsaid lip forms a liquid seal with the upper surface of a test strip wheninserted into said opening; and (c) a raised bump that contacts thebottom surface of a test strip when inserted into said opening; whereinsaid strip holder is configured to at least partially encompass a sampleapplication region of said test strip when inserted into said opening.8. The strip holder according to claim 7, wherein the force applied bysaid lip on said top surface of said strip inserted into said opening issubstantially equal, opposite in direction and at the same location asthe force applied by said bump on said bottom surface of said stripinserted into said opening.
 9. The strip holder according to claim 8,wherein said forces range in magnitude from about 0.01 lb to 0.05 lb.10. A test strip meter, wherein said meter comprises: a test stripholder, wherein said test strip holder comprises: (a) an opening forreceiving a test strip; and (b) a lip associated with said opening,wherein said lip forms a liquid seal with the upper surface of said teststrip when inserted into said opening.
 11. The meter according to claim10, wherein said strip holder is readily removable from said meter. 12.The meter according to claim 10, wherein said strip holder is configuredto at least partially encompass a sample application region of said teststrip when inserted into said opening.
 13. The meter according to claim10, wherein said strip holder further comprises a raised bump thatcontacts the bottom surface of said test strip when inserted into saidopening.
 14. The meter according to claim 13, wherein the force appliedby said lip on said top surface of said strip inserted into said openingis substantially equal, opposite in direction and at the same locationas the force applied by said raised bump on said bottom surface of saidstrip inserted into said opening.
 15. The meter according to claim 14,wherein said forces range in magnitude from about 0.01 lb to 0.05 lb.16. The meter according to claim 10, wherein said meter furthercomprises said test strip inserted into said opening of said stripholder.
 17. A method of using a test strip meter, said methodcomprising: (a) providing a meter comprising a test strip holder,wherein said test strip holder comprises: (i) an opening for receiving atest strip; and (ii) a lip associated with said opening, wherein saidlip forms a liquid seal with the upper surface of said test strip wheninserted into said opening; and (b) inserting said test strip into saidopening.
 18. The method according to claim 17, wherein said strip holderis configured to at least partially encompass a sample applicationregion of said test strip following said inserting.
 19. The methodaccording to claim 17, wherein said strip holder further comprises araised bump that contacts the bottom surface of said test strip uponsaid inserting.
 20. The strip holder according to claim 19, wherein theforce applied by said lip on said top surface of said strip insertedinto said opening is substantially equal, opposite in direction and atthe same location as to the force applied by said bump on said bottomsurface of said strip inserted into said opening.